Bmesh: Immediately encounter strange error after adding new layer and running bmesh op
Assign your int layer before you assign geometry.  PS took out verts from bisectable geometry.

import bpy, bmesh

mesh = bpy.context.edit_object.data
bm = bmesh.from_edit_mesh(mesh)
cut_layer = bm.edges.layers.int.new('cut')
geom = [e for e in bm.edges[:] + bm.faces[:] if e.select]

ret = bmesh.ops.bisect_plane(bm, geom=geom , plane_co=(0,1,0), plane_no=(1, 0, 0))
bm.edges.layers.int.remove(cut_layer)

bmesh.update_edit_mesh(mesh)